In recent years, to meet severer corrosion resistance requirements for automobile bodies, surface treated steel strips in the form of conventional cold rolled steel strips plated with zinc or zinc base alloy are often used.
Such surface treated strips include galvanized steel strips, galvannealed steel strips, zinc electroplated steel strips and zinc base alloy electroplated steel strips. A high bare corrosion resistance is needed in internal strip configurations including internal cavity-defining structures and bends (hems) where protective coatings after pressing and body assembly cannot fully cover.
Organic composite coated steel strips having chromate and organic high molecular weight resin coatings on a zinc or zinc base alloy plated steel substrate were developed for automobile use as disclosed in Japanese Patent Application Kokai Nos. 57(1982)-108292 and 58(1983)-224174. These proposals intend to provide high corrosion resistance by coating a chromate treated, zinc or zinc base alloy plated steel substrate with a coating composition containing an organic resin and a water dispersed silica sol. However, the use of water dispersed silica sol gives rise to several problems including
(1) poor resistance to chromium dissolution due to the presence of residual water-soluble components in the coating which allow water to penetrate into the coating during subsequent chemical conversion treatment so that the chromium in the underlying chromate layer can dissolve out to contaminate the environment,
(2) separation of the coating during alkali degreasing which can lead to a loss of corrosion resistance, and
(3) poor adherence between the resin layer and the chromate layer caused by penetration of water underneath the coating upon exposure to a corrosive environment resulting in dissolution of the water soluble components therein to exhibit high alkalinity cleaving the interfacial bond between the resin and the chromate.
One solution for the above problems is disclosed in Japanese Patent Application Kokai No. 63(1988)-22637 which uses a coating composition comprising an epoxy resin and a hydrophobic silica obtained by subjecting silica on its surface to organic substitution in an organic solvent. Although compatibility is maintained between silica sol and the organic resin and improved adherence after coating is achieved, it has been pointed out that this coating suffers from an insufficient corrosion resistance.
Furthermore, the use of normal silica sol of either a water or an organic solvent system resulted in a problem of a significantly inferior spot weldability compared to zinc base alloy plated steel strip having no organic coating.